$ D = \left[\begin{array}{rr}0 & 2 \\ 1 & -2\end{array}\right]$ $ A = \left[\begin{array}{rrr}2 & 0 & 4 \\ 5 & -2 & 4\end{array}\right]$ What is $ D A$ ?
Because $ D$ has dimensions $(2\times2)$ and $ A$ has dimensions $(2\times3)$ , the answer matrix will have dimensions $(2\times3)$ $ D A = \left[\begin{array}{rr}{0} & {2} \\ {1} & {-2}\end{array}\right] \left[\begin{array}{rrr}{2} & \color{#DF0030}{0} & \color{#9D38BD}{4} \\ {5} & \color{#DF0030}{-2} & \color{#9D38BD}{4}\end{array}\right] = \left[\begin{array}{rrr}? & ? & ? \\ ? & ? & ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ D$ , with the corresponding elements in column $j$ of the second matrix, $ A$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ D$ with the first element in ${\text{column }1}$ of $ A$ , then multiply the second element in ${\text{row }1}$ of $ D$ with the second element in ${\text{column }1}$ of $ A$ , and so on. Add the products together. $ \left[\begin{array}{rrr}{0}\cdot{2}+{2}\cdot{5} & ? & ? \\ ? & ? & ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ D$ with the corresponding elements in ${\text{column }1}$ of $ A$ and add the products together. $ \left[\begin{array}{rrr}{0}\cdot{2}+{2}\cdot{5} & ? & ? \\ {1}\cdot{2}+{-2}\cdot{5} & ? & ?\end{array}\right] $ Likewise, to find the element at row 1, column 2 of the answer matrix, multiply the elements in ${\text{row }1}$ of $ D$ with the corresponding elements in $\color{#DF0030}{\text{column }2}$ of $ A$ and add the products together. $ \left[\begin{array}{rrr}{0}\cdot{2}+{2}\cdot{5} & {0}\cdot\color{#DF0030}{0}+{2}\cdot\color{#DF0030}{-2} & ? \\ {1}\cdot{2}+{-2}\cdot{5} & ? & ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{rrr}{0}\cdot{2}+{2}\cdot{5} & {0}\cdot\color{#DF0030}{0}+{2}\cdot\color{#DF0030}{-2} & {0}\cdot\color{#9D38BD}{4}+{2}\cdot\color{#9D38BD}{4} \\ {1}\cdot{2}+{-2}\cdot{5} & {1}\cdot\color{#DF0030}{0}+{-2}\cdot\color{#DF0030}{-2} & {1}\cdot\color{#9D38BD}{4}+{-2}\cdot\color{#9D38BD}{4}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{rrr}10 & -4 & 8 \\ -8 & 4 & -4\end{array}\right] $